Anomalous dispersion glass exhibiting the anomalous partially dispersing property in the long wave length region

ABSTRACT

This invention provides an anomalous dispersion glass exhibiting an η d value of 1.54-1.71 and anν d value of 55-30, and the anomaly of a partially dispersing property in the long wave length region. The anomalous dispersion glass contains B 2  O 3  45-70 mol.%; Pb0 5-30 mol.%; Na 2  O 1-15 mol.%; SiO 2  0-10 mol.%; Al 2  O 3  5-25 mol.%; one or more of CaO, BaO, MgO and CdO 0-16 mol.%, and one of TiO 2 , ZrO 2 , Sb 2  O 3 , Wo 3  and Tl 2  O 0-3 mol.%. Fluorine may be present in an amount such that the F/O ion ratio in the resulting glass lies in the range of 0-0.35.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part Application of Ser. No. 526,996, filed Nov. 25, 1974, now abandoned, in turn a Continuation-in-Part Application of Ser. No. 316,607, filed Dec. 19, 1972, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical glass exhibiting the anomaly of partial dispersing property in a long wave length region, i.e., the wave length region which is longer than the c line (6263 A).

2. Description of the Prior Art

The partial dispersion ration (Pxy) of a glass for light of two different spectral wavelengths x and y is generally indicated by expression (1): ##EQU1## WHICH IS DETERMINED ON THE BASIS OF THE DIFFERENCE IN THE REFRACTIVE INDEX ηF for the 4861 A line and the refractive index ηc for the 6263 A line. The symbols ηx and ηy represent the refractive indices of the light at wavelengths x and y, respectively. On the other hand, in normal dispersion glasses expression (2) gives the relationship between the partial dispersion ratio of the normal dispersion glass and the Abbe number (νd):

    νD = (ηD - L) (ηF - ηc.)

    Pxy = Axy + bxy.sup.. νd                                (2)

wherein Axy and bxy are constants selected in accordance with the wave length x and y of light, respectively, Axy and bxy being determined by two glasses selected from normal glasses, ηd indicates the refractive index for the d line (5876 A) and ηF and ηc have the same meaning as in expression (1).

When pluralities of such normal dispersion glasses are assembled into e.g., a lens, the chromatic abberation between two lines on a spectrum, for example, between the F line and the C line, can be removed, but the extinction of the secondary spectrum renders super achromatism impossible.

On the other hand, if an anomalous dispersion glass exhibiting the anomaly of partial dispersion in a long wave length region is assembled with another anomalous dispersion glass exhibiting the anomaly of partial dispersion in a short wave length region, super achromatism can be attained over the whole visible light range.

The words "anomalous partial dispersion glass" mean a glass which exhibits a deviation value (Δ Pxy) which is not equal to zero in expression (3) which represents the partial dispersion ration Pxy, wherein Δ Pxy shows the deviation of the partial dispersion ratio from the straight line of that of a normal dispersion glass and "Pxy" is the (relative) partial dispersion of a normal dispersion glass:

    Pxy = Pxy + Δ Pxy                                    (3)

The value of Δ Pxy indicates the degree of anomaly of the partial dispersion. It is known that larger Δ Pxy values result in favorable super achromatism.

In the case that the partial dispersion ratio in a long wave length region is indicated as

    PcA' = ηc - ηA'/ηF - ηc

by selecting the C line of 6263 A and the A' line of 7682 A as x and y, respectively, in expression (1), the increase of the PcA' value and the decrease of the Abbe number γd are effective to increase the degree of the anomaly (Δ PcA'). In this formula, PcA" is the (relative) partial dispersion wherein x equals C and y equals A', ηc is the refractive index wherein x equals c and ηA' is the refractive index wherein y equals A'.

Therefore, a conventional anomalous dispersion glass exhibiting the anomaly of partial dispersion in the long wave length region contains B₂ O₃ as the glass forming oxide since B₂ O₃ is effective to absorb light in the infrared region and increase the PcA' value, and PbO, which absorbs light in the ultraviolet region and decreases the Abbe number.

SUMMARY OF THE INVENTION

We, the inventors, have discovered that the degree of the anomaly of partial dispersion (in the long wave length region) can be increased by decreasing the specific gravity of an anomalous dispersion glass. The glass of this invention was made based on the this discovery and produced by introducing Na₂ O and, optionally, SiO₂, which impart a low specific gravity to the resulting glass, into a B₂ O₃ --PbO glass to thereby increase the anomaly of the partial dispersion. In one embodiment, the Na₂ O is substituted for a portion of the B₂ O₃, say of about 15 mol.%, in a conventional B₂ O₃ -- PbO anomalous dispersion glass. However, when this is done, the concentrations in the glass of the conventional ingredients must still remain within the range set forth in the detailed description of the invention hereinbelow.

A first object of this invention is to provide a remarkable anomalous partial dispersion glass in the long wave length region.

A second object of this invention is to provide a stable glass suitable for mass production.

A third object of this invention is to provide a glass which has good resistance to chemicals.

A fourth object of this invention is to select useful components for compensating the refractive index of such a partial anomalous dispersion glass.

A fifth object of this invention is to provide an anomalous partial dispersion glass in the long wave length region which has a low refractive index.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relationship between the Abbe number (νd) and the partial dispersion ration (PcA') of an optical glass of this invention showing the anomaly of the partial dispersion in the long wave length region.

FIG. 2 shows the relationship between the Abbe number (νd) and the partial dispersing ratio (Phg) of a glass of his invention showing that the Phg value of this glass in the short wave length region is smaller than that of the general normal dispersion glass.

FIGS. 3a, 3b, 4a, 4b, 5a, 5b, 6, 7a, and 7b show the improved properties achieved with the glass of the present invention when Na₂ O is substituted for SiO₂.

FIGS. 8, 9, 10 and 11 relate to the glass composition of Table 6.

DETAILED DESCRIPTION OF THE INVENTION

The PbO which is introduced into the conventional B₂ O₃ --PbO glass to decrease the Abbe number also increases the partial dispersing ratio in the short wavelength region with increased PbO content in the glass. The increase of the PbO content is restricted since the partial dispersing ratio in the short wavelength region of the anomalous dispersion glass in the long wavelength region must be smaller than that of a normal dispersion glass for super achromatism.

On the other hand, in the case that a light component (such as Na₂ O or SiO₂) capable of decreasing the specific gravity of the resulting glass is added to the glass composition to lower the Abbe number, the anomaly of the partial dispersion in the long wavelength region can be increased without causing the above defects. Accordingly, the Abbe number can be advantageously reduced by the combined action of PbO and the introduction of the light weight component.

B₂ O₃ --PbO glasses have poor resistance to chemicals, and the addition of Al₂ O₃ is not completely satisfactory in improving resistance to chemicals, i.e. a B₂ O₃ --PbO--Al₂ O₃ glass, B₂ O₃ dissolves in water or an acid and the chemical durability of the glass is dependent on the B₂ O₃ content. Accordingly, the B₂ O₃ content in such a glass must be reduced to increase the chemical durability, but a reduction in the B₂ O₃ content is not favorable for increasing the PcA' value.

However, if Na₂ O and, optionally, SiO₂ is introduced into such a B₂ O₃ --PbO--Al₂ O₃ glass according to this invention, the reduction of the PcA' value due to the decrease of the B₂ O₃ content can be compensated to some extent. In addition, Na₂ O is not dissolved in acid or water to an extent as great as B₂ O₃, and SiO₂ improves the chemical durability. Therefore, one major feature of this invention is that the remarkable anomaly of the partial dispersion (in the long wave length region) can be obtained by the addition of Na₂ O and, optionally, SiO₂.

In addition, the anomalous dispersion glass of this invention has a low refractive index (an ηd value less than 1.71) due to the Na₂ O and, optionally, SiO₂.

The PbO content in this glass influences the value of the Abbe number, and serves to improve the acid resistance to an extent comparable with Al₂ O₃. Therefore, in the case of controlling the Abbe number by decreasing the PbO content, the reduction of the acid resistance can be compensated for the same extent by the addition of CaO, BaO, MgO or CdO. TiO₂, ZrO₂, WO₃, Sb₂ O₃ or Tl₂ O may be introduced into the glass composition to adjust the optical constant.

The partial dispersion ratio PcA' (in the long wave length region) can be increased by replacing a part of oxygen ions in the glass with F ions, i.e. F ions can be introduced into the glass by replacing a part of the oxide components with fluorides.

The anomalous dispersion glass of this invention was developed based on the above empirical work, and contains B₂ O₃ 45-70 mol.%; PbO 5-30 mol.%; Na₂ O 1-15 mol.%; SiO₂ 0-10 mol.%; Al₂ O₃ 5-25 mol.%; one or more of CaO, BaO, MgO and CdO 0-16 mol.%; and any one of TiO₂, ZrO₂, WO₃, Sb₂ O₃ and Tl₂ 0-3 mol.%, in which a part of the oxide components may be replaced with fluorides so that the F/O ratio (ion ratio) in the resulting glass composition lies in the range of 0-0.35. The F/O ratio means ##EQU2## . This glass exhibits the anomaly of partial dispersion (in the long wave length region) and has an ηd value of 1.54-1.71 and a νd value of 55-30. 1-12 mol.% of CaO is especially effective to improve the acid resistance of the glass but any one component selected from the group of TiO₂, Tl₂ 0.5-3 mol.% is effective to adjust the optical constant.

The content of each component is set to be within the above restricted ranges for the following reasons.

The addition of less than 45 mol.% of B₂ O₃ makes the resulting glass unstable and does not provide the anomaly of partial dispersion in the long wave length region, whereas the addition of more than 70% of B₂ O₃ is detrimental to the chemical durability of the glass .

The addition of more than 10 mol.% of SiO₂ elevates the liquidus temperature of the resulting glass and makes the glass unstable.

The addition of more than 15 mol.% of Na₂ O worsens the chemical durability of the resulting glass, and once worsened, the chemical durability cannot be restored.

The sum of Na₂ O and SiO₂ must exceed 1% to increase the anomaly of the partial dispersion in the long wave length region, and the upper limit is restricted by the maximum content of Na₂ O and SiO₂.

Less than 5 mol.% of Al₂ O₃ makes the resulting glass unstable and more than 25 mol.% of Al₂ O₃ causes insoluble matter in the glass.

Excess amounts of CaO, BaO, MgO and CdO elevate the liquidus temperature of the glass and make the glass unstable.

The addition of excess amounts of TiO₂, ZrO₂, WO₃ or Sb₂ O₃ makes the glass unstable and devitrifies the glass. Too much Tl₂ O is detrimental to the chemical durability of the glass.

PbO is added to the glass composition in an amount of 5-30 mol.% as a balancing component, i.e., to make 100%.

The introduction of excess amounts of F causes opaque matter in the glass due to the precipitation of fluoride crystals. In order to mass produce the glass of the present invention a preferred composition consists of 53-57 mol.% of B₂ O₃, 5-29 mol.% of PbO, 3.5-7.5 mol.% of Na₂ O, 0-4 mol.% of SiO₂, 10.5-17.5 mol.% of AlO₃ and 0-12 mol.% of CaO.

Some examples of glasses within this invention will now be explained with reference to Tables and the drawings.

The partial dispersion ratio of the glass in the long wave length region is indicated as PcA' as described hereinbefore, whereas that in the short wave length region is indicated as Phg (=ηh -ηg/ηF -ηc) by selectingan h line of 4047 A and a g line of 4358 A as x and y, respectively, in expression (1).

The acid resistance of the glass was tested to examine the chemical durability in the following manner:

Samples (1-26) of glasses were finely ground into a size of 28-35 mesh, and the resultant glass particles were washed with ethanol, dried and charged in a platinum cage with a specific gravity gram which was then dipped in 150 cc of a hot nitric acid solution [pH of 2.20] and heated over boiling water for 1 hr, removed and then fully dried.

Thereafter, the weight loss of each glass sample was measured.

The test results are shown in the following Tables:

    __________________________________________________________________________     Table 1                                                                        __________________________________________________________________________              1   2     3     4     5     6                                         B.sub.2 O.sub.3                                                                       53 mol.%                                                                             57    57    57    70    50                                        PbO    29 mol.%                                                                             25    20    15    12.5  10                                        Na.sub.2 O                                                                            7.5 mol.%                                                                            7.5   7.5   7.5   2     15                                        Al.sub.2 O.sub.3                                                                      10.5 mol.%                                                                           10.5  15.5  17.5  12.5  20                                        CaO     --    --    --   3     3     5                                         ηd 1.70130                                                                              1.67216                                                                              1.63648                                                                              1.61067                                                                              1.59314                                                                              1.57797                                   νd  33.43 36.20 39.66 43.10 46.70 45.76                                     PcA'   0.3227                                                                               0.3253                                                                               0.3315                                                                               0.3359                                                                               0.3457                                                                               0.3381                                    Phg    0.5110                                                                               0.5003                                                                               0.4891                                                                               0.4799                                                                               0.4661                                                                               0.4751                                    Specific                                                                              4.06  3.79  3.45  3.17  2.94  2.89                                      Gravity                                                                        Weight less                                                                    by acid(%)                                                                             --   1.48  1.58  1.75  4.36  3.25                                      __________________________________________________________________________     Table 2                                                                        __________________________________________________________________________            7     8     9     10    11                                              B.sub.2 O.sub.3                                                                       60    47    70    57    56                                              PbO    25    10    12.5  10    5                                               Na.sub.2 O                                                                            7.5   13    --    3.5    5.5                                            Al.sub.2 O.sub.3                                                                      7.5   20    12.5  17.5  17.5                                            SiO.sub.2                                                                             --    10    5     3     4                                               CaO    --    --    --    9     12                                              η d                                                                               1.67320                                                                              1.56420                                                                              1.58221                                                                              1.59292                                                                              1.56470                                         νd    36.47                                                                                46.69                                                                                46.04                                                                                47.70                                                                                53.02                                         PcA'    0.3262                                                                               0.3398                                                                               0.3441                                                                               0.3427                                                                               0.3502                                         Phg     0.4967                                                                               0.4705                                                                               0.4654                                                                               0.4674                                                                               0.4563                                         specific                                                                       gravity                                                                               3.80  --    2.88  2.92  2.66                                            weight loss                                                                    by acid (%)                                                                           2.78  --    --    1.77  1.88                                            __________________________________________________________________________     Table 3                                                                        __________________________________________________________________________            12    13    14    15    16    17                                        B.sub.2 O.sub.3                                                                       50    50    50    50    50    50                                        PbO    10    10    10    10    10    10                                        Na.sub.2 O                                                                            8     8     5     10    10    10                                        Al.sub.2 O.sub.3                                                                      20    20    20    20    20    20                                        CaO    --    --    --    2     --    2                                         BaO    12    --    --    --    --    --                                        MgO    --    12    --    8     8     --                                        CdO    --    --    15    --    2     8                                         ηd 1.60112                                                                              1.59410                                                                              1.63335                                                                              1.58788                                                                              1.59133                                                                              1.60490                                   νd  45.92 46.28 44.14 46.22 45.91 44.87                                     PcA'   0.3376                                                                               0.3396                                                                               0.3366                                                                               0.3393                                                                               0.3390                                                                               0.3385                                    Phg    0.4759                                                                               0.4718                                                                               0.4794                                                                               0.4745                                                                               0.4750                                                                               0.4758                                    specific                                                                       gravity                                                                               3.23  2.96  3.40  2.94  2.99  3.14                                      weight loss                                                                    by acid (%)                                                                           1.78  1.82  2.03  1.87  1.97  1.87                                      __________________________________________________________________________     Table 4                                                                        __________________________________________________________________________            18    19    20    21    22                                              B.sub.2 O                                                                             57    57    57    57    57                                              PbO    22    25    18    19    17                                              Na.sub.2 O                                                                            7.5   7.5   7.5   7.5   7.5                                             Al.sub.2 O.sub.3                                                                      10.5  7.5   15.5  15.5  15.5                                            TiO.sub.2                                                                             3     --    --    --    --                                              ZrO.sub.2                                                                             --    3     --    --    --                                              WO.sub.3                                                                              --    --    2     --    --                                              Sb.sub.2 O.sub.3                                                                      --    --    --    1     --                                              Tl.sub.2 O                                                                            --    --    --    --    3                                               ηd 1.67042                                                                              1.69300                                                                              1.63552                                                                              1.63467                                                                              1.63554                                         νd  35.49 35.36 39.43 39.75 38.43                                           PcA'   0.3245                                                                               0.3250                                                                               0.3310                                                                               0.3317                                                                               0.3297                                          Phg    0.5056                                                                               0.5010                                                                               0.4930                                                                               0.4915                                                                               0.4942                                          specific                                                                       gravity                                                                               3.60  --    --    --    --                                              weight loss                                                                    by acid (%)                                                                           2.71  --    --    --    --                                              __________________________________________________________________________     Table 5                                                                        __________________________________________________________________________            23    24    25    26                                                    B.sub.2 O.sub.3                                                                       57    57    57    57                                                    PbO    5     15    25    25                                                    Na.sub.2 O                                                                            --    --    --    --                                                    2NaF   7.5   7.5   7.5   7.5                                                   Al.sub.2 O.sub.3                                                                      17.5  17.5  6.5   1.5                                                   2AlF.sub.3                                                                            --    --    4     9                                                     SiO.sub.2                                                                             5     --    --    --                                                    CaO    8     3     --    --                                                    ηd 1.54950                                                                              1.60229                                                                              1.6575                                                                               1.6543                                                νd  53.71 44.35 37.4  38.0                                                  PcA'   0.3509                                                                               0.3380                                                                               0.327 0.328                                                 Phg    0.4555                                                                               0.4786                                                                               0.498 0.496                                                 specific                                                                       gravity                                                                               2.59  3.11  3.76  3.81                                                  weight loss                                                                    by acid (%)                                                                           2.40  2.62  2.62  2.03                                                  F/O ion ratio                                                                         0.061 0.062 0.181 0.344                                                 __________________________________________________________________________

The relationship between νd and PcA' of each of glass specimens (1-26) is shown in FIG. 1, in which dots 1-26, which correspond to glass specimens (1-26) exist on the left side of line A which shows the relationship between νd and PcA' of a normal dispersion glass. This means that the glasses of this invention have the anomaly of the partial dispersion ratio in the long wave length region.

The relationship between νd and Phg of each of glass samples (1-26) is shown in FIG. 2, in which dots 1-26 corresponding to glass samples (1-26) exist on the left side of line B, which shows the relationship between νd and Phg of a normal dispersion glass. This means that the partial dispersion ratio of the glass of this invention in the short wave length region is smaller than that of a normal dispersion glass.

For example, glass specimen 10 listed in Table 2 was produced in the following manner. A mixture composed of H₃ BO₃ 86.0 g, Na₂ CO₃ 7.2 g, Al(OH)₃ 33.9 g, PbO 8.9 g, Pb(NO₃)₂ 7.4 g, SiO₂ 3.0 g and CaCO₃ 14.9 g was prepared. 100 g of this mixture was blended well and melted in a Pt crucible (120 cc volume) open to the atmosphere at 1350° C for about 1 hour and 30 minutes. The resultant melt was poured in a metal mold, and cooled slowly from the transition temperature of the glass (550° C) to the room temperature (cooling rate about 25° C/hr.).

The glass obtained was reheated to its transition temperature (550° C) and kept at that temperature for 10 hours, slowly cooled down to 450° C with a cooling velocity of about 10° C/hr. and further cooled to room temperature. Specimen 10 having the composition and the optical properties described in Table 2 was thus produced.

The other specimens were produced in a similar manner.

FIGS. 3a to 7b show that the presence of Na₂ O in the glass composition is critical and provides an excellent effect in the anomaly of partial dispersing properties. In these figures, 7 glass compositions are compared to determine the effect of Na₂ O on the optical properties of the glass. The results clearly show that Na₂ O provides a superior and unexpectedly remarkable anomalous partial dispersion glass in the long wave length region.

More particularly, in order to show the criticality of the presence of Na₂ O in the glass composition to provide an excellent effect in the anomaly of partial dispersing property, we have compared the following compositions and obtained the following results:

    ______________________________________                                         Data                                                                           ______________________________________                                         Substitution of SiO.sub.2 to Na.sub.2 O                                        Composition (mol%)                                                                    (1)     (2)     (3)   (4)  (5)   (6)   (7)                              B.sub.2 O.sub.3                                                                       70      70      70    50   50    50    50                               SiO.sub.2                                                                             5       3       0     15   10     5     0                               Na.sub.2 O                                                                            0       2       5      5   10    15    20                               Al.sub.2 O.sub.3                                                                      12.5    12.5    12.5  20   20    20    20                               PbO    12.5    12.5    12.5  10   10    10    10                               ______________________________________                                    

    __________________________________________________________________________     Optical Property                                                               Nd   1.58310                                                                              1.58275                                                                              1.58240                                                                              1.57125                                                                              1.56820                                                                              1.56511                                                                              1.56223                               Vd   46.82 46.68 46.52 48.57 47.59 46.63 45.64                                 PcA' 0.3430                                                                               0.3432                                                                               0.3435                                                                               0.3384                                                                               0.3386                                                                               0.3388                                                                               0.3390                                Δ PcA'                                                                        +0.0106                                                                              +0.0109                                                                              +0.0114                                                                              +0.0041                                                                              +0.0053                                                                              +0.0066                                                                              +0.0078                               __________________________________________________________________________

Explanation of Figures

From FIG. 3-a and FIG. 3-b, it is apparent that ηd value decreases by the substitution of SiO₂ to Na₂ O. As a result, νd=ηd-1/η_(F) -η_(c) also decreases (FIGS. 4-a and 4-b).

On the other hand, PcA' value is constant or slightly increased (FIGS. 5-a and 5-b).

As a result, from FIG. 6 showing νd-PcA' relationship, it is apparent that the partial dispersion ratio (PcA') becomes larger. This is also apparent from FIGS. 7-a and 7-b.

Thus, the above data and figures show that Na₂ O provides more advantageous results in anomalous partially dispersing property in the long wave length region.

In order to illustrate the improvements of the present inventin wherein Na₂ O is employed in an anomalous dispersion glass mainly comprising B₂ O_(3l) , PbO and Al₂ O₃, compositions have been prepared and tested as tabulated in Table 6.

                                      Table 6                                      __________________________________________________________________________     1          2     3     4     5     6    7    8                                 B.sub.2 O.sub.3                                                                     60    57.5  55    52.5  50    59   57   55                                     mol%                                                                      PbO  25    25    25    25    25    26   28   30                                Na.sub.2 O                                                                          7.5   10    12.5  15    17.5  7.5  7.5  7.5                               Al.sub.2 O.sub.3                                                                    7.5   7.5   7.5   7.5   7.5   7.5  7.5  7.5                               ηd                                                                              1.67320                                                                              1.67297                                                                              1.67273                                                                              1.67252                                                                              1.67229                                                                               --   --   --                               νd                                                                               36.47 35.50 34.52 33.55 32.60 35.72                                                                               34.22                                                                               37.72                             PCA' 0.3262                                                                               0.3245                                                                               0.3233                                                                               0.3217                                                                               0.3207                                                                               0.3240                                                                              0.3213                                                                              0.3190                            ΔPCA'                                                                         0.0050                                                                               0.0043                                                                               0.0042                                                                               0.0037                                                                               0.0037                                                                               0.0036                                                                              0.0025                                                                              0.0019                            Da   2.78  2.47  2.10  1.86  1.58   --   --   --                               __________________________________________________________________________

In Table 6 specimen 1 corresponds to specimen 7 of Table 2 hereinabove. Modifications of the composition of specimen 1 are made in the compositions of specimens 2 through 8. In specimens 2 through 5, a portion of B₂ O₃ is substituted by Na₂ O while in specimens 6 to 8, a portion of B₂ O₃ is substituted by PbO for comparison.

From FIG. 9, it is apparent that due to the substitution of B₂ O₃ by Na₂ O the acid proof property Da is improved and νd tends to decrease. Moreover, in FIG. 10, the reduction behavior of νd is illustrated. Accordingly, since the reduction of νd follows the reduction of P_(CA).spsb.' due to the reduction of B₂ O₃, the resulting glasses with difficulty approach the normal line in the relationship of νd and P_(CA).spsb.'.

On the other hand, in case of the glasses obtained by the substitution of B₂ O₃ by PbO, PbO more effectively affects the reduction of νd than Na₂ O, but due to the fact that the reduction of P_(CA).spsb.' is larger than that of the former case, such glasses tend to easily approach the normal line.

That is, as is apparent from FIG. 11, Na₂ O is more effective than PbO for the purpose of reducing as small as possible the deviation ΔP_(CA).spsb.' from the normal line.

FIG. 8 plots Abbe number against partial dispersion ratio for specimens of Table 6, while FIG. 11 plots amount of Na₂ O or PbO substituted for B₂ O₃ against ΔP_(CA).spsb.' for the specimens of Table 6.

Glass compositions have been prepared and tested as set forth in Table 7 to illustrate differences in properties between an anomalous dispersion glass not containing Na₂ O and the same glass containing 2.5 mol.% Na₂ O with corresponding decrease in B and B₂ O₃ content. ΔP_(CA).spsb.' is less with the glass of this invention.

                  Table 7                                                          ______________________________________                                                   9            10                                                      B.sub.2 O.sub.3                                                                            65             67.5                                                PbO         25             25                                                  Na.sub.2 O  2.5            0                                                   Al.sub.2 O.sub.3                                                                           7.5            7.5                                                 ηd      1.67448        1.67560                                             νd       39.55          41.20                                               PCA'        0.3328         0.3364                                              ΔPCA' 0.0084         0.0102                                              Da          3.66           4.60                                                ______________________________________                                    

As particularly described above with respect to the Tables and the drawings, the glass of this invention has a high anomalous partial dispersion property in the long wave length region and is suitable for the production of a super achromatic lens used in optical instruments.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. 

What is claimed:
 1. An anomalous dispersion glass exhibiting an ηd value of 1.54-1.71 νd value of 55-30 and the remarkable anomaly of the partial dispersing property in the long wave length region, which contains B₂ O₃ 45-70 mol%, PbO 5-30 mol%, Na₂ O 1-15 mol%, SiO₂ 0-10 mol%, Al₂ O₃ 5-25 mol%, one or more components selected from the group consisting of CaO, BaO, MgO and CdO 0-16 mol%, and any one component selected from the group consisting of TiO₂, ZrO₂, Sb₂ O₃ WO₃ and Tl₂ O 0-3 mol%, wherein F ion may be added to said oxide mixture in such amount that the F/O ion ratio in the resulting glass lies in a range of 0-0.35.
 2. An anomalous dispersion glass as claimed in claim 1, which consists of 53-57 mol% of B₂ O₃, 5-29 mol% of PbO, 3.5-7.5 mol% of Na₂ O, 10.5-17.5 mol% of Al₂ O₃ and 0-4 mol% of SiO₂.
 3. An anomalous dispersion glass as claimed in claim 1, which, as an additional ingredient contains 1-12 mol% of CaO.
 4. In a process for producing an anomalous dispersion glass comprising mainly B₂ O₃,PbO and Al₂ O₃, the improvement which comprises introducing 1-15 mol% of Na₂ O into said glass while maintaining the amount of B₂ O₃ at 45-70 mol% and the amount of PbO at 5-30 mol%.
 5. The method of claim 4 wherein there is also added to said glass SiO₂ 0-10 mol%, Al₂ O₃ 5-25 mol%, one or more components selected from the group consisting of CaO, BaO, MgO and CdO 0-16 mol%, and any one component selected from the group consisting of TiO₂,ZrO₂, Sb₂ O₃, WO₃ and Tl₂ O 0-3 mol%, wherein F ion may be added to said oxide mixture in such amount that the F/O ion ratio in the resulting glass lies in a range of 0-0.35.
 6. The method of claim 4 wherein 3.5-7.5 mol% of Na₂ O is employed with 53-57 mol% of B₂ O₃, 5-29 mol% of PbO, 10.5-17.5 mol% of Al₂ O₃ and 0-4 mol% of SiO₂.
 7. The method of claim 4 wherein 1-12 mol% of CaO is also added to said glass composition. 